Microporous films have a structure that enables fluids and/or gases to flow through. The effective pore size is at least several times the mean free path of the flowing molecules, namely, from several micrometers and down to about 100 angstroms. Sheets of the microporous films are generally opaque, even when made from an originally transparent material, because the surfaces and internal structures scatter visible light.
Microporous films have been utilized in a wide variety of applications, such as filtration of solids, ultrafiltration of colloids, diffusion barriers, and in cloth laminates. Additional applications include: filter cleaning antibiotics, beer, oils, and bacterial broths; analysis of air, microbiological samples, intravenous fluids and vaccines. Microporous films are also utilized in the preparation of surgical dressings, bandages, and in other fluid transmissive medical applications.
Ion conductive membranes (ICMs) are also being developed from microporous films. Ion conductive membranes have found application in membrane electrode assemblies (MEAs) as solid electrolytes. One specific example application of an MEA is a hydrogen/oxygen fuel cell. The ICM is located between the cathode and anode in the MEA, and transports protons from near the catalyst at the hydrogen electrode to the oxygen electrode thereby allowing the current to be drawn from the MEA. The ICMs are particularly advantageous in these applications as they replace acidic liquid electrolytes, such as are used in phosphoric acid fuel cells, which are very hazardous.
Ion conductive membranes are also used in chloroalkali applications to separate brine mixtures and form chlorine gas and sodium hydroxide. The membranes selectively transport the sodium ions across the membrane, while rejecting the chloride ions. ICMs are also useful in the area of diffusion dialysis where, for example, caustic solutions are stripped of their impurities. The membranes are also useful for their operation in vapor permeation and separations due to their ability to transfer polar species at a faster rate than non-polar species.
The microporous films must have sufficient strength to be useful in these various applications. Often this need for increased strength requires increased membrane thickness, which can impair the utility of the membrane by, for example, decreasing the ionic conductance of ion conductive membranes. Membranes that are inherently weak at small thicknesses (for example less than 0.050 mm) must be reinforced with additional materials causing the final product to have increased thickness.